Electrical windings

ABSTRACT

A winding for electrical inductive apparatus having first and second groups of axially aligned pancake coils of the continuous type. The coils of the first group are spaced and interconnected to increase the series capacitance of the group, adjacent coils of the second group are interconnected, and the two groups are interconnected to provide at least one series path through the winding.

United states Patent 1 Van Nice [54] I ELECTRICAL WINDINGS [75] Robert 1. Van Nice, Sharon, Pa.

Inventor:

Assignee:

Pittsburgh, Pa. 1

Filed: June 8,1971

Appl. No.: 151,116

US. Cl. .;.336/60, 336/70, 336/187 Int. Cl .1101: 27/08, 1101615/14 Field of Search ..336/l85, 69, 70, 186, 187,

[5 6] I References Cited UNlTED STATES PATENTS Westinghouse Electric Corporation, I

Stein ..336/l X Muller ..'336/69 X [45] Jan. 9, 1973 3,299,385 l/1967 Stein ..336/ X 3,477,052 11/1969 Van Nice ..336/70 3,090,022 5/1963 Steinm; ....336/69 X 2,980,874 4/1961 Tarbox ..336/ 3,371,300 2/1968 Stein ..336/70 Primary Examiner-Thomas J. Kozma Att0rney-A. T. Stratton and Donald R Lackey [57] ABSTRACT A winding for electrical inductive apparatus having first and second groups of axially aligned pancake coils of the continuous type. The coils of the first group are spaced and interconnected to increase the series capacitance of the group, adjacent coils of the second group are interconnected, and the two groups are interconnected to provide at least one series path through the winding.

11 Claims, 11 Drawing Figures PATENTEUJAn 9197s 3,710,292

sum 3 UF 4 l ssnn 24 25 26 27 28 29 30 3| 32 33 34 35 Q L f 15 FIG.4.

E lllllllllllfi KJIIIHIIIILla PATENTEUJAN ems 3.710.292

SHEET l [1F 4 IE I? ELECTRICAL WINDINGS BACKGROUND OF THE INVENTION winding constructed of continuous pancake coils, when spaced and connected according to the teachings of the prior art, has the disadvantage of distributing surge voltages in a very non-uniform manner. A lightning or switching surge produces large voltage differences between the turns of the line end pancake coil, and between axially spaced pancake coils at the line end. Increasing the thickness of the insulation at these locations to accommodate these high surge stresses would at first seem to be the most logical solution, until it is realized that the non-uniform voltage distribution is due to the relatively low through or series capacitance of the winding, compared with the capacitance of the winding to ground. Therefore, increasing the thickness of the turn insulation, and increasing the spacing between coils at the line end, reduces the series capacitance of this portion of the winding causing a still greater proportion of a voltage surge to concentrate at this location. Thus, increasing the dimensions of the insulation at the line end of the winding is at least partially self defeating and the space factor of the winding, i.e., the ratio of conductive volume to total volume of the winding, is deleteriously affected, which in turn increases the dimensions and weight of the associated magnetic core.

Shielding the pancake coils located adjacent to the line end, or ends of the winding, reduces the capacitance of the winding to ground and thus improves the surge voltage distribution pattern across the winding, but shielding adds conductive material which is not used to carry useful current, and it adds the insulation required to electrically insulate the shielding members, resulting in a reduced space factor and increased costs.

Pancake coils of the interleaved turn type, wherein the pancake coils are wound with at least two radially interleaved conductors, with the conductors connected such that electrically adjacent turns are physically separated by a turn from an electrically distant portion of the winding, greatly increase the series capacitance of the winding and thus reduce'the electrical stresses at the line end, or ends of the winding. Interleaved windings also provide an excellent space factor since all of the conductive material is-used to carry useful current. However, they are more costly to manufacture than the continuous type of pancake coil. Thus, new and improved transformer winding constructions are continually sought which have a manufacturing cost more nearly approaching that of the continuous winding, and. surge characteristics which approach that of the interleaved or shieided winding.

My co-pending application Ser. No. 96,0l0, filed Dec. 8, 1970, which is assigned to the same assignee as the present application, discloses a winding construction for core-form power transformers which utilizes a group of interleaved turn pancake coils at the line end, or ends, of the winding assembly, with the balance of the coils being of continuous type construction. The

degree of interleaving selected for the group of interleaved turn pancake coils is such that the mis-match between the series capacitance of the interleaved group and the continuous group is reduced, to limit the magnitude of voltage oscillation due to the mis-match. This arrangement produces a relatively low cost winding assembly because the majority of the pancake coils are of the conventional continuous construction, with the in.- terleaved group, or groups, dropping any surge voltage to a magnitude which may be safely applied to the continuous group of coils. While this arrangement produces a lower cost winding than one which utilizes I all interleaved turn pancake coils, it would be desirable to eliminate even the relatively few interleaved turn coils from the structure, if this could be accomplished without undue sacrifice in surge performance. The interleaved turn pancake coils of my hereinbefore mentioned co-pending application require double or triple interleaving to reduce the series capacitance to the proper range, which increases the complexity and cost of these coils, and these higher degrees of interleaving become impractical when it is necessary to provide more than one series path between the ends of the winding.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved winding for electrical power transformers of the coreform type, which has a plurality of axially aligned continuous pancake coils. A group of coils adjacent to the line end, or ends, of the winding are divided into pairs, with the coils of each pair being closely adjacent to one another to increase the capacitance between them. The coils in this group, or groups, are interconnected within the group to provide a voltage difference between axially adjacent turns of each pair which is substantially uniform across the coil builds, and which has a magnitude at least equal to the voltage drop across one coil of the pair. Predetermined ends of adjacent pancake coils of the remaining pancake coils are interconnected to provide at least one series electrical path therethrough, and thus at least one series path is connected to the group, or groups, of coils at the line end, or ends, of the winding. These line end groups have a higher series capacitance than the remaining coils, resulting in a more uniform distribution of surge voltages across these sections, and they function to drop the surge voltage to a magnitude suitable for application to the remaining continuous pancake coils of the windingstructure. While the series capacitance of the end group, or groups, is higher than that the conventional continuous winding, it is not so high as to cause damaging voltage oscillations due to the mis-match of series capacitance between connected groups.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood, and further advantages and uses thereof more readily apparent, when considered in view of the following detailed high voltage winding shown in FIG. 1;

FIG. 2 is a graph which compares the surge voltage distribution pattern across conventionally constructed windings using continuous pancake coils, with a winding constructed according to the teachings of the invention;

FIGS. 3, 4 and 5 are fragmentary, elevational views, in section, of a transformer winding which illustrates other embodiments of the invention which may be used to construct the high voltage winding of the transformer shown in FIG. 1;

FIG. 6 is a fragmentary, elevational view, in section, of a transformer winding which illustrates the embodiment of the invention shown in FIG. 1, except modified to include two series paths through the winding;

FIG. 7 is a fragmentary, elevational view, in section, of a transformer winding constructed according to still another embodiment of the invention, which embodimentprovides a higher steady-state voltage between adjacent turns of predetermined coils than the previous embodiments of the invention;

FIG. 7A is a schematic diagram of the winding shown in FIG. 7;

FIG. 8 is a fragmentary elevational view, in section, of a winding constructed according to another embodiment of the invention; and

FIG. 9 is a fragmentary elevational view, in section, of a transformer, winding constructed according to voltage. The pancake coils of high voltage winding 16 Y are axially divided into first, second and third groups 30, 32 and 34, respectively, with all of the coils of the three groups being of the continuous type, i.e., the physically adjacent turns are from electrically adjacent positions in the winding.

The first group of pancake coils, which group is connected to line terminal 26, includes continuous pancake coils 36, 38, 40 and 42; the thirdgroup 34, which group is connected to line terminal 28, includes continuous pancake coils 44, 46, 48, and 50; and, the second group 32, which group is connected between the first and third groups 30 and 34, respectively, includes a plurality of continuous pancake coils, with eight pancake coils 52, 54, 56, 58, 60, 62, 64 and 66 being illustrated and the balance indicated generally by broken line 68. High voltage windings which have only one end connected to a line or elevated potential, and the other end adapted for connection to ground or to the neutral of a wye configuration, would not require the third group. 34 of pancake coils, as the end of second group 32 which is now connected, to the third group 34, would be connected to the ground or neutral terminal. I

Broadly, the invention is a new and improved winding structure which utilizes continuous coils in line end sections, spaced and connected to act as surge voltage dropping impedances for a plurality of additional contron.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown a partial sectional elevation of a transformer 10, which embodies the teachings of the invention. Transformer 10 is a power transformer of the core-form type. It is filled to a predetermined level with an insulating an cooling fluid dielectric, such as mineral oil or askarel, and it may be either single or polyphase. Since each phase of the transformer would be similar, in the event it is polyphase, only one phase is illustrated in FIG. 1 in order to simplify the drawing.

Transformer 10 includes a magnetic core 12, which may be of conventional construction, including a winding leg 14 having an axis 20 about which high and low voltage windings 16 and 18, respectivelyfare concentrically "disposed. Low voltage winding 18, which may be of conventional construction, has a plurality of conductor turns 22 insulated from the magnetic core 12 and the high voltage winding 16 by electrical insulating means 24. High voltage winding 16 includes a plurality' of pancake or disc type coils, axially aligned in a stacked arrangement with their openings in alignment, about the axis of the magnetic core leg 14. High voltage winding 16 has first and second terminals or ends'26 and28, respectively, and for purposes of example, it willbe assumed that each end is adapted for'conn'ectinuous pancake coils. The line end sections are con-' structed to increase their series capacitance to a point where these sections may be subjected, to incoming surge voltages without undue stress concentration between the turns of the line end pancake coil, and between the first few line end pancake coils. The maximum attainable series capacitance using continuous pancake coils isnot so great as to provide a mismatch of series capacitance between connected groups which would result in harmful voltage oscillations. I

The line end group, or groups, depending upon whether the winding has one or two line terminals, is constructed to increase the' 'series capacitance of the group by dividing the group into pairs of coils, and disposing the coils of each pair with their major surfaces closely adjacent one another. Then, the coils in the group are interconnected toprovide a substantiallyuniform voltage difference between axially adjacent conductor turns in a pair, across the coil build, to more uniformly stress the insulation between the conductors of which the coils of the pair are wound. Further, in

' tion to an elevated electrical potential, i.e., to a line order to increase the effective series capacitance of the pair by increasing energy stored between the coils of a I ing application Ser. No. 150,994, filed concurrently with this application, in the name of D. A. Yannucci,

which application is assigned to the same assignee as the present application. Since the first and third groups 30 and 34 are of like construction, it will only be necessary to describe the first group in detail.

Group 30 includes the necessary number of pairs of coils which are required to drop a surge voltage to a magnitude which may be safely applied to the coils of the second group 32. In this embodiment, the minimum number of pairs is one, since only two coils are required to complete the basic interconnection arrangement, but in most applications two or three pairs of coils will be required per group.

Pancake coils 36 and 38 are grouped to provide ,a first pair 70, and pancake coils 40 and 42 are grouped to provide a second pair 72. The coils of each pair are disposed with their major surfaces closely adjacent one another, to increase the capacitance between them, and, as illustrated in FIG. 1, a thin insulating washer member 74, formed of pressboard, or the like, is preferably disposed between the coils of each pair, with the coils of the pair being butted tightly against opposite sides of the insulating washer member. In order to reduce the spacing between the coils of a pair to a minimum, the insulating washer member preferably has a thickness dimension of only about 0.030 inch. The function of this insulating washer member is to provide a high electrical impulse breakdown strength between the two coils, and also to prevent the insulation of the turns of the two coils from being damaged when the coils are pressed tightly together. Reducing the spacing between coils 36 and 38 reduces the thickness of the electrical insulation between them, compared with conventional prior art spacing dimensions, but it also increases their series capacitance, which reduces the magnitude of impulse electrical stresses created between them when the winding is subjected to impulse voltage. The structure described using the thin solid insulating washer member between coils of a pair has been found to reduce the impulse stresses more than the breakdown strength of the insulation between the coils has been reduced by the reduced spacing between the coils. Thus, the disclosed arrangement is effective in reducing the spacing between the coils and thus increasing their series capacitance, without suffering an offsetting disadvantage of too little electrical breakdown strength in the insulating material between the coils.

In like manner, coils 40 and 42 of the second pair 72 are closely spaced by a thin insulating washer member 76.

Adjacent pairs of coils are spaced to provide cooling ducts between them for removing heat from the coils, such as duct 78 between pairs 70 and 72. Ducts having a width dimension of about 0.25 inch have been found to be suitable.

The coils within the first group 30 are interconnected to provide at least one series path therethrough, and to provide a steady-state voltage difference between axially adjacent turns in each pair which is substantially uniform across the builds of the coils which make up a pair, and with the magnitude of the voltage difference being at least as great as the steady-state voltage across either one of the coils of the pair. This unifonnly stresses the electrical insulation disposed between the coils of a pair, utilizing it most efficiently, and it increases the electrical charge stored between coils of a pair.

More specifically, the first pancake coil 36 is constructed, in this embodiment of theinvention, of a single insulated conductor, such as copper or aluminum, which conductor spirals inwardly, and, as is characteristic of continuous coils, the physically adjacent conductor turns are from electrically adjacent positions in the winding, with the turn number, starting from the line terminal 26, being indicated onthe turn. The innermost end of pancake coil 36, and of all of the pancake coils of the winding, regardless of where the electrical circuit first enters the coil, will be called the start, and the outermost end will be called the finish. Pancake coil 38 is of like construction, also spiralling inwardly, and in the same circumferential direction as pancake coil 36. The circuit direction is indicated by arrows in FIG. 1A, which is a schematic diagram of high voltage winding 16 shown in FIG. 1.

The finish of pancake coil 36 is connected to line terminal 26 via conductor 51, the start of pancake coil 36 is connected to the finish of pancake coil 38 via conductor 53, and the start of pancake coil 38 is connected to the start of pancake coil 40 via conductor'55. Therefore, the first two pancake coils, i.e., those of the first pair 70, are interconnected with a start-finish connection 53, and the second and third pancake coils, i.e., adjacent pancake coils of adjacent pairs, are interconnected with a start-start connection 55. The interconnection of the two pancake coils of a pair is thus completed before proceeding to the next pair, which, as illustrated in FIG. 1A, provides a voltage difference of one voltage unit between axially adjacent turns of the pancake coils of a pair. The uniformity of the voltage difierence between axially adjacent turns of pair may also be observed from FIG. 1, as there is a voltage difference, in the example, of 12 turns between any two axially adjacent turns.

The finish of pancake coil 40 is connected to the start of pancake coil 42 via conductor 57, and the finish of pancake coil 42 is connected to the finish of the first pancake coil 52 of the second group 32, via conductor 59. Thus, the third and fourth 'pancake coils, i.e., those of the second pair 72, are interconnected with a finishstart connection 57 and the fourth pancake coil 42 is connected to the fifth pancake coil with a finish-finish connection 59. This arrangement creates a single electrical path in the first group, which spirals inwardly through coil 36, inwardly through coil 38, outwardly through coil 40, and outwardly through coil 42. The coils of the first and second pairs, while spiralling inwardly and outwardly, respectively, are wound to direct the circuit in the same circumferential direction in all of the coils, in order to provide an additive magnetomotive force in the magnetic core.

In the second group 32, the coils are all axially spaced to provide cooling ducts between them, and adjacent pancake coils are interconnected with start start, finish-finish connections across the group. The

electrical path spirals inwardly and outwardly in the same'circumferential direction, from coil to coil, to provide an additive magnetomotive force in the magnetic core. A suitable dimension for the ducts between adjacent pancake coils of the second group is about 0.25 inch.

An electrical winding was constructed according to the teachings of FIG. 1, which will be hereinafter referred to as the FIG. 1 winding, except the third group 34was not used. Two pairs of coils were used in the line end group 30, and 54 coils were used in the second group 32, using 0.030 inch pressboard between coils of a pair, and 0.25 inch ducts between adjacent pairs of the first group, and between adjacent pancake coils of the second group. A similarly mated electrical winding was constructed using conventional continuous winding design, having 0.25 inch ducts between all pancake coils, with this winding being hereinafter referred to as the prior art winding. Both windings were placed in mineral oil and subjected to a chopped wave surge of 520 KV (450 KV BIL.) with three microsecond chopping, and the voltage distribution observed. FIG. 2 is a graph which compares the results obtained. The ordinate of the graph plots the voltage measured between pancake coils in each winding, in percent of the applied surge voltage, and the abscissa indicates 'coil location, with the coils being numbered consecutively starting at the line end of each winding. The measured values for the prior art windings were plotted and connected with a line to provide a curve' 80, and the measured values for the FIG. 1 winding were plotted and connected to provide a curve 82.,

The FIG. 1 winding substantially dropped the voltage stress which appeared across the line end pancake coils (curve 82), compared with the voltage street which appeared stress or between the line end coils using the prior art winding (curve 80), from about 35 to percent of the applied wave between the first two coils,

from about 28 to percent between the second andthird coils, and from about to 10 percent between the third and fourth coils. The voltage distribution across the second group of coils of the FIG. 1 winding, which group is similar in construction to the prior art'winding tested to obtain curve, 80, is much better than can be explained by the fact that the first group of paired coils actedas'a surge voltage dropping impedance. From an oscillogram taken when the 520 KV chopped wave was applied to the FIG. 1 winding, a chopped wave of similar configuration appeared at the junction of the first and second groups, i.e., between coils 4 and 5, with this chopped wave having a magnitude. of 390 KV. Upon applying a chopped wave having a magnitude of 390 KV to the line end coil of a prior art continuous winding, one does not obtain the low stresses shown on curve 82 starting at point 84, obtained when the FIG. 1 winding was tested, but a curve which approximates curve 80 starting at point 86. Thus, the first group of coils of the FIG. 1 winding provides the function of a surge voltage dropping impedance, enabling a lower B.I.I design to be used for the second group of coils than would otherwise be possible, and the first group of coils of the FIG. 1 winding also provides an unexplainable reduction in stresses across the second group of pancake coils which enables a still lower B.I.L. design to be used for'the second group of coils than would be expected by using the surge voltage dropping characteristic of the first group alone as the design criterion for the second group.

FIGS. 3, 4 and 5 are fragmentary elevational views, in section, of other embodiments of the invention in which the voltage between axially adjacentturns of a coil pair is substantially the same as the voltage across one of the coils of the pair. Like reference numerals in FIGS. 1, -2, 3 and 4 indicate like components, and modified components which are similar in function to the components of FIG. 1, are given like reference numerals but with prime marks to distinguish-'modified structure.

More specifically, FIG. 3 illustrates a winding 16 which is similar in construction to winding 16 shown in FIG. 1, except each pair of coils in the first group v are of like construction, each having a start-finish connection between the first and second coils of the pair, instead of a start-finish connection between the coils of alternate pairs, and a finish-start connection between I the coils of intervening pairs. Thus, the first pair of coils 70 in winding 16 is constructed and connected as hereinbefore described relative to FIG. 1, and the second pair'72 is constructed and interconnected in the same manner as the first pair 70. Instead of a finish-' start connection 57 between pancakes and 42, a start-finish connection 57' is used between pancake coils 40' and 42', which thus necessitates a start-start connection 55' between the starts of pancake coils 38 and 42', instead of connection 55 between the starts of pancake coils 38 and 40 shown in FIG. 1, and a finishfinish connection 59' between coils 40 and 52, instead of a finish-fmish connection 59 between pancake, coils 42 and 52, as shown in FIG. 1. 7

FIG. 4 illustrates a winding 16" in which the line terminal enters the start of the first pancake coil, instead of the finish, and with all finish-start connections being used between adjacent pancake coils throughout the winding. Terminal 26 is connected to the start of pancake coil 36" via conductor 51", the finish of pancake coil 36" is connected to the startof pancake coil 38" via conductor 53", the 'finish of pancake coil 38 is connected to the start of pancake coil 40 via conductor 55", the finish of pancake coil 40, is connected to the start of pancake coil 42' via conductor 57", and the finish of pancake coil 42" is connected to the start of pancake coil 52" via conductor 5 9' FIG. 5 illustrates a winding 16" which is similar to the winding 16" shown in FIG. 4, except the coils of the second pair 72" are interconnected with a startfinish connection 57', instead of afinish-start connection 57", the finish-start connection 55"interconnects the finish of pancake coil 36" with the start of pancake coil 57", and the finish-start connection 59" interconnects the finishof pancake coil 40" with the start of pancake coil 52". v

FIG. 6 is a fragmentary elevational view in section, of

a winding 90, which is similar in construction to wind ing 16 shown in FIG. 1, but instead of winding each.

pancake coil with asingle insulated conductor, two insulated conductors are used which are wound such that their turns are radially interleaved around one another, but unlike the process of interleaving turns to increase the series capacitance of a coil, the side-by-side or radially adjacent turns of the two conductors are "at the same electrical potential, with the higher series group 92 including pancake coils 96, 98, 100 and 102,

such that the turns are radially adjacent one another, providing two electrical paths through each coil which are termed the A and B paths, with the turn number and path letter being indicated on each turn of the pancake coils shown in FIG. 6. The first two pancake coils 96 and 98 are grouped and disposed in closely adjacent relation, separated only by a thin insulating washer member 112, and the third and fourth pancake coils 100 and 102 are placed closely-adjacent one another, separated only by the thin insulating washer member 1 14. The adjacent pairs 108 and 110 are separated by a cooling duct 116, which may be about .25 inch in dimension.

Pancake coils 96 and 98 of the first pair 108 both spiral inwardly, the pancake coils 100 and 102 of the second pair 110 both spiral outwardly, and then the pancake coils of the second group 94 spiral inwardly and outwardly, from coil to coil, across the second group. The finish ends of the A and B conductors are connected to line terminal 118 via conductors 120 and 122, the start ends of the A and B conductors of the first pancake coil 96 are connected to the finish ends of the A and B conductors of the second pancake coil 98, via conductors 124 and 126, respectively, the start ends of the A and B conductors of the second pancake coil 98 are connected to the start ends of the A and B conductors of pancake coil 100 via start-start conductors 128 and 130, the finish ends of the A and B conductors of pancake coil 100 are connected to the start ends of the A and B conductors of pancake coil 102 via conductors 132 and 134, and the finish ends of the A and B conductors of pancake coil 102 are connected to the finish ends of the A and B conductors of pancake coil 104. As illustrated in FIG. 6, the positions of the A and B conductors may be transposed from coil to coil, in order to reduce the magnitude of losses due to circulating current in the two parallel connected paths through the winding.

In all of the embodiments described to this point, the

' coils of each pair are interconnected before proceeding to the next pair of pancake coils, resulting in the steady state voltage difference between axially adjacent turns of a pair being equal to the voltage across one coil of a pair. The invention, however, is not to be so limited, as the first group may be constructed such that there is more than one voltage unit appearing between axially adjacent turns of a pair. For example, the construction disclosed in British Pat. 1,099,491 may be used. FIGS. 7 and 8 are fragmentary elevational views, in section, of windings constructed according to an embodiment of the invention in which the line end groups utilize the construction disclosed in the British patent, resulting in two voltage units between axially adjacent turns of a pair, i.e., twice the voltage across one coil of a pair.

More specifically, FIG. 7 illustrates a winding 140 constructed according to the teachings of the invention, having first and second groups 142 and 144 of continuous type pancake coils. A first group 142 includes pancake coils 146, 148, 150 and 152, while the second group 144 includes a plurality of pancake coils, with only two coils 154 and 156 being illustrated in order to simplify the drawing. The first two coils of the first group are placed in closely adjacent relation to form a first pair 158, and the third-and fourth coils 150 and 152 are placed in closely adjacent relation to provide a second pair 160. The coils of the first pair 158 are separated only by a thin insulating member 162, and the coils of the second pair 160 are separated only bya thin insulating member 164, as hereinbefore described relative to the embodiment of the invention shown in FIG. 1.

- The pancake coils of the first pair 158 spiral inwardly in a first circumferential direction, and the pancake coils of the second pair 160 spiral outwardly in a circumferential direction opposite to that of the first pair, but instead of being interconnected as taught in the FIG. 1 embodiment, they are interconnected such that four pancake coils are required to complete one basic interconnection pattern. The finish of pancake coil 146 is connected to line terminal 156 via conductor 168, the start of pancake coil 146 is connected to the start of pancake coil 150, via start-start connection 170, the finish of pancake coil 150 is connected to the finish of pancake coil 148, via finish-finish connection 172, the start of pancake coil 148 is connected to the start of pancake coil 152, via startstart connection 174, and the finish of pancake coil 152 is disconnected to the finish of pancake coil 154, via finish-finish connection 176.

FIG. 7A is a schematic diagram of the winding 140 shown in FIG. 7, illustrating that two voltage units are created between adjacent coils of a pair, and the uniformity of this voltage difference between axially adjacent turns may be observed by comparing the turn numbers of axially adjacent turns of coils in a pair, as H- lustrated in FIG. 7.

FIG. 8 is a fragmentary elevational view which illustrates a modification of the winding 140 shown in FIG. 7, with like reference numerals in FIGS. 7 and 8 indicating like components, and with like reference numerals except for a prime mark indicating modified components. Instead of following the sequence shown in FIG. 7 of traversing the first, third, second and fourth pancake coils, the pancake coils in the FIG. 8 embodiment are connected to traverse the first, fourth, second, and third pancake coils of the first group 142'. The finish of pancake coil 146 is connected to line terminal 166 via conductor 168, the start of pancake coil 146 is connected to the start of pancake coil 152', via conductor 170', the finish of pancake coil 152' is connected to the finish of pancake coil 148, via conductor 172; and the start of pancake coil 148 is connected to the start of pancake coil 150', via conductor 174. The finish end of pancake coil 150' is connected to the finish of pancake coil 154, via conductor 176'.

FIG. 9 is a fragmentary elevational view, in section,

of a winding constructed according to still another em-- bodiment of the invention. The embodiment of the invention shown in FIG. 9 is a multiple conductor embodiment, as was the embodiment shown in FIG. 6, but instead of winding two discrete conductors together in each coil of the first group, as taught by the embodiment of FIG.6, each pancake coil in this embodiment of the invention is formed of a single conductor, with the voltage difference between axially adjacent turns of the pancake coils of each pair being one voltage unit.

More specifically, FIG. 9 illustrates a winding 180 having aplurality of pancake coils arranged in first and second groups 182 and 184, respectively, with the first group including pancake coils 186, 188, and 192,

and with the second group 184 including a plurality of Y pancake coils, only two of which, 194 and 196 are illustrated.

The first two pancake coils of group 182 are disposed closely adjacent one another to provide a first pair 198, separated by a thin insulating washer member 199, and the third and fourth pancake coils are disposed closely adjacent one another to provide a second pair 200, separated by a thin insulating washer member 201. Pancake coils 186 and 188 spiral inwardly, pancake coils 190 and 192 spiral outwardly, and then the coils spiral inwardly and outwardly, from coil to coil, across the second group of coils, in conventional continuous coil construction. The finish of the first pancake coil 186 is connected to terminal 202 via conductor 204, and this circuit, termed theA circuit, spirals inwardly through the first pancake coil 186. The start of pancake coil 186 is connected to the start of pancake coil 192, via conductor 206. The finish of pancake coil 192 is connected to the finish of pancake coil 194, via conductor 208. Unlike the pancake coils of the first group, the pancake coils of the second group are wound with the A and B conductors radially adjacent one another. Thus, the finish of pancake coil 192 is connected to the A conductor of pan-cake coil 194.

The start of pancake coil 190 is connected to line terminal 202, via conductor 210, the finish of pancake coil 190 is connected to the finish of pancake coil 188, via conductor 212, and the start of pancake coil 188 is connected to the finish of the B conductor of pancake coil 194, via conductor 214. Pancake coil 190 thus utilizes single conductor pancake coils in first group 182, and multiple conductor pancake coils in the second group 184. I

In summary, there have been disclosed new and improved high voltage electrical windings for power transformers of the core-form type, which combine the surge voltage distribution advantage of shielding or interleaving with the attractive cost of continuous type windings. All of the pancake coils of the disclosed windings are of the continuous construction, with a group of pancake coils adjacent eachline end of the winding having the coils arranged in pairs, and with the coils of each pair being disposed with their major surfaces as closely together as possible to increase the capacitance between the coils of a pair. The adjacent pairs are conventionally spaced to provide cooling ducts, and the remaining coils are also spaced to provide cooling ducts/This combination of groups provides a substantially lower stress across the line end coils, it reduces the voltage applied to the group of coils adjacent to the line end coils, with this reduction being substantially greater than can be explained by the surge voltage dropping function of the line end group alone. Thus, the windings of the invention may be constructed more economically than shielded or interleaved windings of the prior art, while obtaining surge voltage distribution characteristics which approach those of shielded and interleaved constructions. The disclosed arrangement is particularly attractive when multiple conductors. must be used in highcurrent windings, as

' the processof interleaving becomes more complicated I claim as my invention: v 1. A winding for electrical inductive apparatus, comprising: an axially aligned stack of pancake coils, said coils being of the continuous type, each having a plurality of insulated conductor turns which define first and second major opposed surfaces,

a first group of coils, starting at one end of the stack, being divided into pairs, with the adjacent major surfaces of the coils in each pair being closely adjacent one another to increase the capacitance between them, and with adjacent pairs being spaced to provide cooling ducts,

means interconnecting the coils of the first group to provide at least one electrical path therethrough, and a voltage difference between axially adjacent turns in each pair which is substantially-uniform across the coil builds, and which has a magnitude at least as great as the voltage across one of the coils of the pair, with said means directly interconnecting the coils of each pair such that the start end of one coil is connected to the finish end of the other coil,

-a second group of pancake coils, starting at the end of the first group, the pancake coils of said second group being spaced in an arrangement which differs from the arrangement of said first group,

means interconnecting predetermined ends of adjacent pancake coils of the second group to provide at least one electrical path therethrough,

and means interconnecting the first and second groups to provide at least one electrical path therethrough.

2. The winding of claim 1 wherein the start-finish connection interconnects like positioned coils in each pair.

3. The winding of. claim 1. wherein the start-finish connection proceeds from the first to the second coil of one pair, and from the second to the first coil in the ad jacent pair.

4. The winding of claim 1 wherein the start of the first coil of the first pair is adapted for connection toan I elevated electrical potential.

5. The winding of claim 1 wherein the finish of the first coil of the first pair is adapted for connection to an elevated electrical potential.

6. The winding of claim 1 wherein the adjacent coils of adjacent pairs are interconnected.

7. The winding of claim 1 wherein like positioned coils of adjacent pairs are interconnected. I

8. The winding of claim 1 wherein the first two pairs of coils are interconnected. with a start-start connection.

9. The winding of claim 1 wherein the first two pairs of coils are interconnected with a finish-start connection. r

10. The winding of claim 1 wherein eachcoil ineludes first and second discrete conductors, the turns 11. The winding of claim 1 wherein an insulating washer member is disposed between each pair of pancake coils of the first group; with the coils of each pair 

1. A winding for electrical inductive apparatus, comprising: an axially aligned stack of pancake coils, said coils being of the continuous type, each having a plurality of insulated conductor turns which define first and second major opposed surfaces, a first group of coils, starting at one end of the stack, being divided into pairs, with the adjacent major surfaces of the coils in each pair being closely adjacent one another to increase the capacitance between them, and with adjacent pairs being spaced to provide cooling ducts, means interconnecting the coils of the first group to provide at least one electrical path therethrough, and a voltage difference between axially adjacent turns in each pair which is substantially uniform across the coil builds, and which has a magnitude at least as great as the voltage across one of the coils of the pair, with said means directly interconnecting the coils of each pair such that the start end of one coil is connected to the finish end of the other coil, a second group of pancake coils, starting at the end of the first group, the pancake coils of said second group being spaced in an arrangement which differs from the arrangement of said first group, means interconnecting predetermined ends of adjacent pancake coils of the second group to provide at least one electrical path therethrough, and means interconnectIng the first and second groups to provide at least one electrical path therethrough.
 2. The winding of claim 1 wherein the start-finish connection interconnects like positioned coils in each pair.
 3. The winding of claim 1 wherein the start-finish connection proceeds from the first to the second coil of one pair, and from the second to the first coil in the adjacent pair.
 4. The winding of claim 1 wherein the start of the first coil of the first pair is adapted for connection to an elevated electrical potential.
 5. The winding of claim 1 wherein the finish of the first coil of the first pair is adapted for connection to an elevated electrical potential.
 6. The winding of claim 1 wherein the adjacent coils of adjacent pairs are interconnected.
 7. The winding of claim 1 wherein like positioned coils of adjacent pairs are interconnected.
 8. The winding of claim 1 wherein the first two pairs of coils are interconnected with a start-start connection.
 9. The winding of claim 1 wherein the first two pairs of coils are interconnected with a finish-start connection.
 10. The winding of claim 1 wherein each coil includes first and second discrete conductors, the turns of which are radially interleaved, with the means interconnecting the coils of the first group, the coils of the second group, and the two groups, providing at least two electrical paths through the first and second groups, said means directly interconnecting the coils of each pair of the first group such that the start ends of the first and second conductors of one coil are connected to different finish ends of the two conductors of the other coil.
 11. The winding of claim 1 wherein an insulating washer member is disposed between each pair of pancake coils of the first group, with the coils of each pair being butted tightly against the insulating washer member to place the two coils of each pair closely together. 